Close coupled gas atomization of liquid metal is being developed as a process for forming metal powders. The close coupled gas atomization process is performed by a nozzle comprised of a melt guide tube extending axially through a cylindrical gas plenum. The cylindrical gas plenum has an inner chamber in communication with an annular orifice, or an annular array of discrete orifices, so that a gas flow therethrough produces an atomizing gas jet which may be comprised of an array of discrete jets. The gas jet has a conical shape converging below the melt guide tube. A stream of liquid metal passing through the melt guide tube and exiting therefrom is atomized by the conical gas jet converging in the stream.
When the gas atomization of liquid metal is commenced, there is an opportunity to view the atomization of the liquid metal from viewports in the atomization chamber. In the atomization process, the atomizing gas flows at supersonic speeds resulting in great scattering and recirculation of the particulate formed by the atomization process. Soon after the atomization starts producing powdered material, recirculating powder from the atomization process obscures the view. In fact, the observation of the atomization nozzle is obscured within seconds after the process is started.
Information regarding the interaction between the atomizing gas and the liquid metal in an atomization zone below the nozzle can be obtained at the start of the atomization process, and before the viewing path to the atomization zone is obscured by the recirculating powder produced by the atomization process. However, it has not been possible to view the atomization process for more than a few seconds after the atomization has begun. The ability to observe the interaction that occurs in the atomizing zone at and below the nozzle tip is lost. For example, one problem that can occur during the atomization process is liquid metal freezing in the melt guide tube, herein referred to as freeze-off. Often the freeze-off cannot be predicted or prevented resulting in undue delay and losses in the atomization process.
Several important properties of metal powder, and the products formed from consolidation of the powder, are dependent on the as-atomized particle size. These properties include composition homogeneity, mechanical performance, e.g. strength, and toughness, as well as physical characteristics of the powder itself, e.g., particle shape, porosity, and flow qualities. Most of these properties improve as particle size decreases, however, powder handling becomes more complicated for finer powder because of caking, environmental contamination, pyrophorosity and other affects.
The strong dependence of properties on particle size translates into an increased demand for atomization process control that provides a predetermined particle size range, and minimizes the production of powder having a particle size above or below the predetermined range. At the same time, a number of variables necessarily change during the atomization process, such as the flow rate of molten metal through the nozzle as the static head pressure of the liquid metal in the crucible changes, temperature increase or decrease, and nozzle wear or constriction. As a result, a series of adjustments can be required during the atomization process in response to the changing variables. We have found that atomization process control can be improved by viewing the interaction of the atomizing gas jet and the liquid metal stream in the atomization zone. Such viewing can also be used to improve the prevention of freeze-off in the atomization nozzle.
U.S. Pat. No. 4,656,331, discloses an infrared sensor suitable for use in detecting the temperature of particles entrained in a plasma spray jet. The sensor is used to control the electrical power input to the plasma torch to heat the particles to their melting temperature prior to impact on a target substrate. U.S. Pat. No. 5,047,612, discloses an apparatus having an infrared sensor for controlling the deposition of a powder in a plasma spray process. A control means responsive to the infrared sensor selectively adjusts a carrier gas flow rate in a powder injection means for the plasma spray apparatus to selectively move the location of a powder impact point on a target. The infrared sensor generates an image representative of a temperature distribution of the powder deposited on the target, the sensor having means for identifying a location of an impact point of the powder upon the target.
An aspect of this invention is to provide an apparatus and method for monitoring and controlling the liquid metal atomization process.
Another aspect of the invention is to provide an apparatus and method for controlling the liquid metal atomization process by viewing the atomization zone, and adjusting as necessary one or more parameters, including the atomization gas pressure, to prevent freeze-off in the melt guide tube or provide a preselect powder size.